There is an increasing polarisation between how the best and worst chains of academies in England are helping their poorest pupils to achieve and progress at school. Our new research has found a growing gap between the most effective chains, where the results of the most disadvantaged students continue to improve, and the least effective chains, which have got worse.

The sponsor academies programme was intended to alter the fortunes of struggling schools in some of the nation’s poorest communities. Started by Labour in 2000, it was intended that helping these schools with the support of philanthropic, educational and business partners would improve the educational outcomes and future lives of young people from the least privileged backgrounds.

Since then, the size of the academies programme has increased dramatically. Five years ago there were about 200 academies, today there are more than 4,000. With the post-election pledge by Nicky Morgan, the secretary of state for education, to turn many “coasting” schools into academies, the new government shows no sign of slowing down its expansion.

The Sutton Trust charity has just published Chain Effects 2015, a report that I have written with lead-author Merryn Hutchings, and Philip Kirby, looking at how well 34 different chains of academies are doing in providing for their disadvantaged pupils.

Best got better, worse got worse

The report includes an index comparing the chains’ 2014 performance for disadvantaged pupils on the most important attainment measures. These include the percentage achieving five A*-C grade GCSEs or equivalent (including English and maths) and the percentage making expected progress in English and maths. It also includes the students’ performance in the English Baccalaureate – five core academic subjects – and their overall performance on their best eight GCSEs. This index updates the version we published in our first report on academy chains in 2014.

The good news is that in 2014, 11 of the academy chains we analysed outperformed the national average. Across the sponsored academies – those sponsored by business or charities rather than deciding to convert to academy status to gain more autonomy – the proportion of disadvantaged students achieving five good GCSEs is at least 15 percentage points higher than the average for disadvantaged students in mainstream schools.

However, the impact of the other 23 academy chains is patchy at best. Our findings show that 44% of the academies analysed were below the government’s new “coasting” level in 2014 and 26 chains had at least one “coasting” school, illustrating the capacity issue to be addressed should the government pursue this measure. The difference between the best-performing and worst-performing chains seems to be increasing too: some of those chains identified as having low results and no improvement in our 2014 report falling back further in the last year.

How to raise the bar

Struggling schools should be supported by a range of suitable improvement agencies, including successful academy chains, maintained school federations, an outstanding local school partner where one exists, or successful local authority provision. Success against robust criteria should be the basis for appointment to provide school improvement, rather than the type of provider.

Data from the Department of Education shows that only 25 out of 704 applications to become a sponsor of an academy had been declined as of November 2014, or 3.6% of the total. Perhaps it is unsurprising, then, that far from all are thriving. We need tighter and more transparent criteria for commissioning sponsorship of a school.

I suggest that the criteria for an academy chain to take on a new school should be four-fold. First, quality – in terms of attainment, and what schools offer to students is key. Second, the chain’s capacity is important. Third should be the strength of a chain’s track record against transparent criteria. Fourth, a chain’s strategic model and educational vision should also be clear, including the governance model, school improvement strategy, regional coherence, and envisaged rate of expansion.

Clearly, for those sponsors that are new and don’t have a track record, it would be especially important that the other three elements are particularly robust and well scrutinised.

As greater numbers of sponsors enter the system, it will also be vital that the mechanisms to remove failing sponsors also become more robust and systematically applied. Our evidence highlights this is necessary to ensure school improvement, and this will be especially vital if sponsorship is to be the main vehicle through which the government plans to improve schools.

Five-year, not seven-year contracts

The current seven-year contract, or funding agreement, between academy chains and the Department for Education should be reduced to five years. Seven years is far too long for a school to remain in the hands of a sponsor that does not secure improvement. In the US, the contracts for or “charters” for their charter schools are typically for three to five years. Evidence to the UK parliament’s Education Select Committee inquiry on academies and free schools suggested that these tight contracts, coupled with rigorous assessment and non-renewal where necessary, are key to success.

We do of course have evidence of some sponsor chains with exceptionally strong success, realising the transformative impact on the educational outcomes of their disadvantaged students originally envisaged by the policy. There is not yet firm evidence on why some academy chains are more successful than others, but it is imperative that steps are taken to ensure that we learn from the successful sponsors, and spread these lessons across the system.